Symmetric Tree

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

1   / \  2   2 / \ / \3  4 4  3

 

But the following is not:

1   / \  2   2   \   \   3    3

Note:

Bonus points if you could solve it both recursively and iteratively.

confused what "{1,#,2,3}" means? 

 

DFS。

1 /** 2  * Definition for a binary tree node. 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     bool check(TreeNode* left,TreeNode* right)13     {14         if(!left && !right) return true;15         if(!left || !right) return false;16         return (left->val==right->val)&&check(left->left,right->right)&&check(left->right,right->left);17     }18     bool isSymmetric(TreeNode* root) {19         if(!root) return true;20         return check(root->left,root->right);21     }22 };